Determining the height or level of a body of liquid within a tank, lagoon, clarifier, etc., can be accomplished in various ways, depending upon the application. For example the height can be determined by direct measurement, e.g., immersion of a graduated rule into the liquid body. This is a common practice for determining the amount (volume) of a liquid, e.g., gasoline, stored in underground tanks. In cases where the liquid is stored in above ground tanks or other vessels it is a common practice to utilize a sight glass with the tank so that the liquid level within the sight glass (and which corresponds to the level within the tank) can be seen.
Indirect measuring systems have also been used in the prior art. One such type of system makes use of a pressure transducer arranged to be submersed at the bottom of the liquid within the body, tank, lagoon, etc., to provide an electrical output signal which represents the pressure produced on the transducer by the liquid. Using this signal, and knowing the specific gravity of the liquid, one can calculate or otherwise determine the height of the liquid (and from that information and with other information known calculate the volume of liquid in the body).
While such an indirect approach to liquid level measurement has advantages over direct measurement approaches for various applications, it nevertheless suffers from a significant drawback, namely, the specific gravity of the liquid must be known. Thus, this approach cannot be used in those applications where the specific gravity of the body of liquid is unknown or cannot be readily determined, thereby severely limiting its applicability.
There is a liquid level guaging device available which provides an electrical signal indicative of the height of a body of liquid irrespective of the specific gravity of that liquid. That device is sold under the trade designation METRITAPE by Metritape, Inc. of Littleton, Mass. The METRITAPE liquid level guaging device does not rely on liquid pressure measurement to determine the liquid's height. Rather it makes use of an elongated, somewhat complex, electrical circuit disposed within a compliant sheath. The sheath is arranged to be submerged vertically in the liquid to be measured so that the sheath extends from a predetermined point at or near the bottom of the tank to a point above the maximum height which the liquid may reach in the tank. The electrical circuit is configured and arranged so that the pressure applied to the sheath by the liquid causes the portion of the sheath disposed within the liquid to collapse, whereupon the electrical resistance of the submerged portion of the sheath changes in direct proportion to the height of the liquid. Accordingly, the electric output signal provided by the METRITAPE device represents the height of the liquid.
While the METRITAPE device appears generally suitable for its intended purposes, it leaves much to be desired from a number of standpoints, e.g., expense, complexity of construction, ruggedness, etc.